PSI - Issue 79

C. Bellini et al. / Procedia Structural Integrity 79 (2026) 233–238

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Fatigue crack growth tests were performed using a computer-controlled servohydraulic testing machine. For the experimental tests, a load ratio of 0.1 was employed. The other experimental conditions were a loading frequency of 30 Hz, a sinusoidal waveform and room temperature environment. To accurately evaluate the crack length, a compliance method was implemented. This technique uses the relationship between the specimen deformation and the length of the crack. The procedure involved using a double cantilever mouth gage to precisely measure the crack opening displacement (COD), that was correlated to the actual crack length using established calibration equations. For validation, the crack length measurements from the compliance method were checked using a standard optical method. 3. Results In this paragraph, Paris' curves for the tested material are described. The graph, reported in Fig. 2, plots the fatigue crack growth rate da/dN as a function of the stress intensity factor range Δ K on a log-log scale. The specimen D, in green, was the worst performer, with its curve positioned to the left and above the others, while the specimens A, B, and C, represented in blue, red, and yellow, respectively, performed significantly better than D. In fact, their curves were shifted noticeably to the right, indicating superior resistance to crack growth. The specimen A appeared to offer the best performance, even if the difference with B and C was not so high. It must be remembered that, in the standard Paris plot, a curve positioned lower and to the right indicates superior material performance, since a higher driving force Δ K is needed for the crack to grow at the same rate.

Fig. 2. Fatigue crack propagation curves for the tested specimens.

To better explain the fatigue crack growth behaviour, SEM images of the fracture surfaces of the broken specimens were obtained and are presented in Fig. 3. The micrographs relevant to the A and B specimens displayed relatively uniform and flat fracture surfaces, that are characteristic of a trans-granular fracture mode. The absence of significant defects like pores or unmelted powder indicates that process parameters used for fabricating A and B specimens, achieved high material density and effective metallurgical bonding. This type of surface is expected from a high-quality additively manufactured part undergoing stable fatigue crack growth. The surface of the C specimen appears rougher and more topographically complex than A and B. Moreover, a lack of fusion defect can be observed on the left of the image, as denoted by the very smooth surface of that zone. The fracture surface of the D specimen is dominated by large, spherical and semi-sintered particles, and large smooth areas. These are unambiguous indicators of lack-of-fusion (LoF) porosity. LoF defects occur when the energy input during the EB PBF process is insufficient to fully melt the metal powder particles, leaving voids in the final part. The LoFs are widely regarded as the most detrimental type of defect for fatigue life. Due to their size and often sharp geometry,